The present invention pertains to method and apparatus for measuring optical quality of an eye. In particular, the present invention pertains to method and apparatus for measuring refractive errors of an eye based on wavefront measurement.
As is well known, a wavefront type refractor is an optical device for measuring refractive errors of an eye based on wavefront measurement. Such a wavefront type refractor can provide comprehensive measurement of the refractive errors of an eye, including high order refractive errors. In addition, such a wavefront type refractor may provide more accurate measurement of the refractive errors of an eye than a conventional auto-refractor. Advantageously, the wavefront measurements can be used to guide refractive laser surgery to correct detected refractive errors. In addition, such a wavefront type refractor may be used to provide prescriptions for eyeglasses and contact lenses.
One implementation of a wavefront type refractor that is well known in the art uses a xe2x80x9cHartmann-Shackxe2x80x9d sensor to measure the wavefront of a light beam generated from an illumination spot projected on the retina and passed through the eye""s optics. As is well known, in such a wavefront type refractor, a probe beam from a laser or a superluminescent diode is projected onto the retina through the eye""s optics. Light scattered by the retina passes through the eye""s optics, and emerges through the eye""s pupil. The wavefront of the emerging beam carries refractive information relating to the eye""s optics. For example, if the eye is emmetropic (i.e., the eye""s optics is without refractive error), the wavefront of the emerging beam should be flat. Relay optics relays the wavefront emerging from eye""s pupil onto the Hartmann-Shack sensor. The Hartmann-Shack sensor measures the distortion of the wavefront to determine the refractive errors of the eye due to aberrations of the eye""s optics.
As is well known, a Hartmann-Shack sensor comprises a lenslet array and a CCD camera located at the focal plane of the lenslet elements of the array. Whenever a beam of radiation to be measured is projected onto a Hartmann-Shack sensor, the lenslet array breaks the beam into sub-apertures, and forms a pattern of focal spots (the pattern of the focal spots carries the signature of the wavefront of the beam to be measured). The CCD camera records the pattern of focal spots, and a computer analyzes the pattern to reconstruct the wavefront of the beam.
As one can readily appreciate from the above, the accuracy of wavefront measurement provided by the above-described wavefront type refractor depends on precise measurement of the positions of the focal spots. Good image quality of the Hartmann-Shack focal spots is thus an essential requirement of such a wavefront type refractor. To resolve the positions of the focal spots precisely, the spots need to be kept to a certain size to cover a predetermined number of pixels in the CCD camera.
One problem encountered in using a Hartmann-Shack sensor to fabricate a wavefront type refractor relates to the defocusing power of an eye. In particular, the defocusing power of an eye varies from patient to patient, and this defocusing power variation can significantly change the spot size of the probe beam on the retina. Consequently, the focal spot size on the Hartmann-Shack CCD camera can change significantly. Another problem encountered in using a Hartmann-Shack sensor to fabricate a wavefront type refractor relates to diffused scattering from a retina. In particular, diffused scattering from the retina produces a bright background for the Hartmann-Shack focal spots, and as a result, reduces image contrast. As is well known, diffused scattering form the retina is a result of the layer structure of the fibers of the retina (the layer structure serves as a two-dimensional wave-guide to enhance lateral diffusion of scattered light).
As one can readily appreciate from the above, a need exists in the art for method and apparatus for measuring refractive errors of an eye that improve upon wavefront type refractors using a conventional Hartmann-Shack sensor.
Embodiments of the present invention advantageously satisfy the above-identified need in the art, and provide method and apparatus for measuring refractive errors of an eye that improve upon wavefront type refractors using a conventional Hartmann-Shack sensor.
Specifically, one embodiment of the present invention is an apparatus for measuring refractive errors of an eye which comprises: (a) a source of a probe beam; (b) a first Badal lens system adapted to project the probe beam into a subject""s eye to form an illumination spot on a retina; (c) a second Badal lens system adapted to image the illumination spot onto an image plane substantially conjugate to the retina; and (d) a spatial filter disposed in the image plane adapted to transmit at least a portion of the image. Advantageously, such an embodiment provides: (a) more accurate wavefront measurement when using a Hartmann-Shack sensor by improving a Hartmann-Shack image; (b) a probe beam spot size that is substantially independent of the defocusing power of the eye; and (c) that a minimal of diffused scattering from the retina falls onto the Hartmann-Shack image of a Hartmann-Shack sensor.